Fatty acid alkyl esters (mono alkyl esters of fatty acids such as methyl and ethyl esters of fatty acids) derived from vegetable oils or animal fats are known to be used as biodiesel. Known processes to produce such fatty acid alkyl esters comprise transesterification of triglycerides included in vegetable oils or animal fats in presence of an alcohol and a catalyst. It is well known in the art to use catalysts such as acid catalysts or more commonly alkali metal catalysts, e g sodium and potassium hydroxide or a metal alkoxide such as sodium or potassium methoxide. In fact, a metal alkoxide is a compound formed by the reaction of an alcohol with an alkali metal. In summary, the transesterification reaction can generally be described as follow:

Among vegetable oils or animal fats that can be used are coconut oil, palm oil, seed oil, olive oil, sunflower oil, Soya oil, rapeseed oil and tallow. Suitable alcohols that can be used are aryl alcohols such as methanol, ethanol, propanol and butanol. Due to low cost, polarity and short chain, methanol is normally used in biodiesel production. Due to high activity, an alkali catalyst such as potassium hydroxide is usually used on industrial scale.
A well-known process for producing fatty acid alkyl esters comprises heating, for example, a vegetable oil to a temperature of normally between 30° C. and 110° C. The process further comprises adding an alcohol and a catalyst to the heated oil. The reaction will result in two phases, one including produced glycerol and the other including produced fatty acid alkyl ester. In addition, the process comprises removing the produced glycerol phase from the produced fatty acid alkyl ester phase.
Fatty acid alkyl esters produced according to known processes as described above normally comprises to high levels of impurities to comply with regulatory norms concerning fatty acid alkyl esters, e g the European norm EN-14214 for fatty acids methyl esters, which especially limits the impurities originating from the metals K, Na, Mg and Ca. Accordingly, there is a need for purification of the produced fatty acid alkyl esters. Known processes for purification include steps of neutralization and washing with water, which will wash out the impurities and produce fatty acid alkyl ester that complies with the regulatory norms and thus can be used as biodiesel. After each step of washing, the water can be separated from the fatty acid alkyl esters by any method known in the art such as await for the mixture to settle into two phases and thereafter drain off the water phase, or by centrifugation of the mixture. The total amount of water used can typically range from 20 to 100% water of the produced volume of fatty acid alkyl esters. After purification is completed, the traces of water comprised in the fatty acid alkyl esters have to be removed, for example, by re-heating the fatty acid alkyl esters. In addition, the water normally contains impurities that in turn needs to be removed, e g by filtrating the water and/or running the water through an ion exchange medium. The water can then be re-used or simply treated as wastewater.
A drawback with known procedures for purification of fatty acid alkyl esters by using water is that it is difficult to reduce the amount of metal impurities, e g calcium and magnesium impurities, to levels that comply with the regulatory norms concerning fatty acid alkyl esters, even when large amount of water is used for purification. Another drawback with known procedures of purifying the fatty acid alkyl esters using water is that the manufacturing of biodiesel, for example, requires high energy input and becomes time-consuming and costly, since the process, for example, requires a large amount of water for purification, which in turn needs to be purified.